Anisotropic, fluorine-based plasma etching method for silicon

ABSTRACT

A method for anisotropic plasma etching of laterally defined patterns in a silicon substrate is described. Protective layers made of at least one silicon compound with a second reaction partner that is entirely compatible with the chemistry of the etching process are deposited before and/or during plasma etching onto the sidewalls of the laterally defined patterns.

FIELD OF THE INVENTION

The invention proceeds from a method for anisotropic plasma etching oflaterally defined patterns in a silicon substrate.

BACKGROUND OF THE INVENTION

The configuration of patterns, for example recesses, in a siliconsubstrate using the plasma etching method is known. It is also known,for example for applications in micromechanics, to use fluorinecompounds for anisotropic plasma etching. The fluorine radicalsgenerated in the plasma act isotropically be with respect to silicon,however; i.e. the lateral etching rate corresponds substantially to thevertical one, which leads to correspondingly large mask undercuts androunded profile shapes. In order to achieve a vertical sidewall with anetching method that uses fluorine compounds, precautions mustadditionally be taken in order to protect the sidewall selectively frometching attack, and to confine the etching to the pattern front, i.e.the bottom of the recess. Discrimination between the sidewall of therecess and the etching front is attained via a highly oriented verticalincidence of energetic ions, which are produced simultaneously in theplasma alongside the chemically active neutral radicals. The ions strikethe surface of the substrate, the etching front being bombarded stronglyby ions but the sidewalls of the recess, on the other hand, onlyrelatively weakly. It is known to use polymer-forming gases such asCHF₃, which are mixed directly with the fluorine-supplying etching gas,as the protective mechanism for the sidewalls. A polymer layer isdeposited onto the sidewall from the polymer-forming monomers present inthe plasma, while the fluorine radicals produced in the plasma at thesame time etch the silicon substrate on the etching front, which ispolymer-free because of the incidence of ions. It is disadvantageousthat intensive recombination occurs, in the plasma and on the way to thesubstrate being etched, between unsaturated polymer-forming monomers andthe fluorine radicals. To overcome this disadvantage, it is known toprevent the disruptive recombination of unsaturated polymer-formingmonomers and the fluorine radicals capable of silicon etching byseparating the plasma etching process into etching steps in whichexclusively fluorine-supplying gases are used, and deposition steps inwhich exclusively deposition gases, such as the polymer-forming gases,are used. Because they are used in the plasma on a temporally separatedbasis, the two varieties of gas used do not encounter one another, sothat appreciable recombination also cannot occur.

It is also known to passivate the sidewalls by using in the plasma,alongside the etching fluorine radicals, oxygen radicals or nitrogenradicals which convert the silicon of the sidewall at the surface intosilicon oxide or silicon nitride, respectively. Since the dielectricsurface is etched particularly strongly by the fluorine radicals withion assistance and less strongly without ion assistance, etchingproceeds essentially on the etching front, while the sidewall remainsrelatively protected. One substantial disadvantage of this method isthat the silicon oxide or nitride layers generated at the surface are ofonly atomic thickness, i.e. are on the order of 1 nm thick or less. Thesilicon oxide or nitride layers generated at the surface therefore donot seal very well, and offer only incomplete protection. The result isthat process control becomes more difficult, and that the process resultis greatly influenced by secondary effects. The profile shapes of thepatterns to be formed are never completely vertical, since sidewallattacks and thus also mask edge undercuts always occur. Cryogenicmethods are used in order to increase the effectiveness of thispassivation: cooling the silicon substrate to temperatures as low as−100° C., in addition to oxygen or nitrogen passivation, “freezes out”the sidewall reaction. This method is described in U.S. Pat. No.4,943,344. Disadvantages include the highly complex equipment and thecosts associated therewith, as well as the comparatively poorreliability of the components.

SUMMARY OF THE INVENTION

The present invention concerns a method for anisotropic plasma etchingof laterally defined patterns in a silicon substrate, protective layersmade of at least one silicon compound being deposited before and/orduring plasma etching onto the sidewalls of the laterally definedpatterns. In a particularly advantageous exemplary embodiment, thepresent invention provides for depositing silicon oxide layers and/orsilicon nitride layers onto the sidewalls of the laterally definedstructures, in particular beams, trenches, combs, or tongues. Thepatterns are preferably defined with the aid of an etching mask. Theprocedure according to the present invention leads, advantageously, to athick (i.e. from several nm to several tens of nm thick) silicon oxideor silicon nitride layer on the sidewalls of the pattern. Even at roomtemperature, this protective layer withstands the etching attack of theradicals formed in the plasma, in particular the fluorine radicals thatare preferably used, and thus makes possible a particularly reliableetching operation which is resistant to malfunction. Advantageously, themethod can also be carried out at lower substrate temperatures; aspecific parameter range of the gas composition leading to theproduction of vertical etching profiles is to be used at each substratetemperature.

The present invention advantageously provides for the etching gas whichreleases fluorine radicals in the plasma to be sulfur hexafluoride SF₆or nitrogen trifluoride NF₃, optionally as a mixture together withargon. In order to make available the components which form theprotective layer, oxide formers and/or nitride formers as well as asecondary reactant are added to the etching gas which supplies thefluorine radicals. Oxygen O₂, dinitrogen oxide N₂O, a different nitrogenoxide NO, NO_(x), carbon dioxide CO₂, or nitrogen N₂ are added as oxideformers or nitride formers. The invention advantageously provides, whenNF₃ is used as the etching gas, for no separate nitride formers to beused, since the nitrogen released during dissociation of the etching gasNF₃ serves for nitrification. Advantageously, silicon tetrafluoride SiF₄is used as the secondary reactant, i.e. as the compound which suppliesthe silicon component of the protective layer. According to the presentinvention, the reaction product SiO₂, Si_(x)N_(y), or a mixture made upof Si_(x)O_(y)N_(z), is deposited onto the sidewall of the pattern fromthe secondary reactant, i.e. preferably SiF₄, and the reaction partner(oxygen or nitrogen) deriving from the oxide former or nitride former.The secondary reactant SiF₄ does not react with the fluorine radicalsderiving from SF₆ dissociation, but only with oxygen or nitrogen,fluorine radicals in fact additionally being released(SiF₄+O₂<==>SiO₂+4F*; SiF₄+xO*<==>SiO_(x)F⁴⁻+xF*).

It is also possible to separate the plasma etching according to thepresent invention into etching and deposition steps that are separatedfrom one another, only etching being conducted during the etching step,and the deposition of the silicon compound described above beingperformed during the deposition step. In particularly preferred fashion,the etching steps are performed alternately with the deposition steps.

The affinity of the oxide formers or nitride formers preferred accordingto the present invention for the secondary reactant, i.e. preferablySiF₄, is low enough that in the gas phase, in particular under the lowprocess pressures that are preferred according to the present inventionand the process conditions provided according to the present invention,especially the excess of free fluorine radicals in the plasma, noappreciable reaction takes place between the oxide formers or nitrideformers and the secondary reactants, even with high-density plasmaexcitation. The advantageous result is to prevent any reaction fromalready occurring in the gas phase, for example between an oxide formerand SiF₄, to form SiO₂; the resulting solid would precipitate onto thesurface of the substrate and might result there in micromasking withetching front roughness, or in needle formation. The low processpressure provided according to the present invention results, by way oflong free path lengths, in a reduction in reaction probability in thegas phase, and in a reduction in microloading effects during siliconetching in narrow trenches.

The present invention thus advantageously leads to the deposition ofetch-resistant silicon compounds on the sidewalls of the laterallydefined patterns, which act as a protective layer. In the course of themethod according to the present invention, activated silicon fluoridecompounds and oxygen radicals or nitrogen radicals, as well as a highconcentration of fluorine radicals, continuously strike the siliconsurface. The result is to form a thick dielectric layer at thoselocations where the intensive ion effect advantageously providedaccording to the present invention does not occur, i.e. on thesidewalls. On ion-bombarded surfaces, i.e. at the bottom of thepatterns, in particular of recesses, or at the etching front, theinfluence of the fluorine radicals and the etching reaction predominate,so that the silicon substrate is removed there. The present inventionthus makes available a passivating system for the sidewalls of laterallydefined patterns that is compatible with the reactive fluorine radicalswithout particular additional measures.

In an exemplary advantageous embodiment, the present invention providesfor carrying out the method using a high-density plasma source, forexample PIE (propagation ion etching), ICP (inductively coupled plasma),or ECR (electron cyclotron resonance), thus making it possible to usehigh flow rates of etching and passivating species as well as low-energyions. A high etching rate and mask selectivity are thereby obtained,ultimately leading to high silicon removal accompanied by low maskmaterial removal.

The present invention advantageously provides for accelerating thebreakdown of silicon compounds, preferably SiO₂, which are possiblydeposited on the etching front by adding so-called SiO₂ scavenger gasessuch as CHF₃, C₄F₈, CF₄, C₂F₆, or C₃F₆ to the gas mixture. Because oftheir carbon content, the SiO₂ scavenger gases etch SiO₂ particularlywell with simultaneous ion action. A clean etching front and even higheretching rates are thus obtained. Needle formation on the etching frontis moreover prevented. These scavenger (i.e. oxide-consuming) gases canbe added continuously to the plasma as a constant admixture, or can beadmitted from time to time or periodically over a short period so as toremove, during a “flash” of this kind, oxide contaminants on the etchingfront.

The present invention provides in particularly advantageous fashion forplasma etching to be carried out with simultaneous ion irradiation, theion energy used preferably being 1 to 100 eV, in particular 30 to 50 eV.

The media used for plasma etching preferably have gas flow rates of 10to 200 sccm at a process pressure of 1 to 50 μbar.

In a further advantageous exemplary embodiment, the present inventionprovides for plasma generation to be accomplished by microwaveirradiation or high-frequency irradiation, at power levels of 500 to2000 W.

The present invention provides, in particular, for the ion density, theion energy, and the ratio between charged and uncharged particles to becontrolled independently of one another.

In an exemplary advantageous embodiment, the present invention providesfor the gas flow rate for SF₆ to be set at 20 to 200 sccm.

The present invention also provides for the gas flow rate for SiF₄ to beset at 10 to 50 sccm.

In a further exemplary embodiment, the present invention provides forthe gas flow rate for oxygen to be set at 10 to 100 sccm, and the gasflow rate for the SiO₂ scavenger gases, in particular C₄F₈, to be set at2 to 10 sccm for a continuous gas flow. With a pulsed scavenger gasflow, a higher value can be selected for this flow rate, for example 30to 60 sccm C₄F₈ every 30 to 60 seconds, for a period of 5 seconds eachtime. Pulsing of the scavenger flow introduces into the process shortcleaning steps which, with a correspondingly higher flow, yield ashort-term intensive cleaning effect without interfering with theprofile shape. Since the scavenger is present only briefly, it cannotdisadvantageously influence the etching profile, but it can neverthelesseffectively remove contaminants from the etching front. On the etchingfront, the intensive ion incidence causes contaminants to be removedmore is quickly than the scavenger can create a breakdown through the(protective) sidewall oxide.

In a further embodiment, the present invention provides for ahigh-frequency power of 5 to 50 W to be made available for ion itacceleration at the substrate electrode, corresponding to accelerationvoltages of 20 to 150 V.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a longitudinal section through a silicon substrate withlateral patterns.

FIG. 2 shows an illustration of the principle of the etching process.

DETAILED DESCRIPTION

FIG. 1 shows a pattern, produced using the plasma etching methodaccording to the present invention, in a silicon substrate.

It shows a substrate 1 having a recess 2 defined by sidewalls 3. Alsodepicted is etching front 4 and a narrow recess 2′.

The method for anisotropic plasma etching of laterally defined recesses2, 2′ in a silicon substrate 1 was carried out with the etching gas SF₆and a gas flow rate of 75 sccm (SF₆). O₂ with a gas flow rate of 38 sccmwas used as the oxide former, and SiF₄ with a gas flow rate of 38 sccmas the secondary reactant. Deposition of the protective layer, i.e. thesilicon compound SiO₂, automatically occurred simultaneously with theplasma etching. The temperature of substrate 1 was 10° C. The processpressure was 20 μbar, and the PIE source that was used supplied amicrowave power level of 650 W (2.45 GHz). To generate an ionacceleration voltage, a high-frequency power level of 5 W (13.56 MHz)was used at the substrate electrode, the ion acceleration energy (DCbias) being 40 V.

FIG. 1 shows that with the process conditions recited above, verticalsidewalls 3 of recesses 2, 2′ were advantageously produced in substrate1. In particularly advantageous manner, extremely small differences inetching rate occur between wide recesses 2 and narrow recesses 2′.

FIG. 2 depicts the principle of the etching process. Identical parts aregiven the same reference numbers as in FIG. 1. Substrate 1, into which alateral recess 2′ is being introduced by the etching process, isdepicted. For passivation of the sidewalls, silicon tetrafluoride SiF₄and oxygen, which ensure fluorine-compatible sidewall passivation, areintroduced into the plasma in addition to the fluorine radicals andpositively charged ions formed therein. At etching front 4, ionassistance results in the reaction of silicon and fluorine radicals toform volatile silicon tetrafluoride, which departs from the etchingfront; this represents the desired etching reaction. Etching occursspontaneously at etching front 4, and thus requires no ion assistanceper se. The high ion incidence there, however, suppresses the formationof silicon oxides or oxyfluorides which inhibit etching.

On sidewalls 3, on the other hand, which are exposed only to arelatively small ion bombardment, a reaction can take place betweensilicon tetrafluoride and oxygen to yield an etch-inhibiting siliconoxide or silicon oxyfluoride, which deposits onto sidewalls 3 as a film.A small proportion of the silicon tetrafluoride formed on the etchingfront, which is attempting to depart from the trench, is consumed in aback-reaction for film formation on sidewalls 3, as indicated by thedashed lines in FIG. 2. The major proportion of the silicontetrafluoride required to form the sidewall film is supplied, however,from the plasma chemistry, i.e. silicon tetrafluoride is introduced intothe plasma as the essential passivating gas together with oxygen, asindicated in FIG. 2 by the solid lines. As is evident from FIG. 2, thisfilm-forming reaction in fact additionally causes the SiF₄ to releasefluorine, which can additionally assist the etching reaction on theetching front. The microloading and RIE lag, i.e. the decrease inetching rate in narrow trenches as compared with wide ones, arerelatively moderate because additional fluorine is formed in the trenchby the wall film-forming reaction.

In a further exemplary embodiment, the following advantageous parameterswere found under ICP (inductively coupled plasma) excitation conditionsusing high-frequency excitation. Gas flow rates were 40 sccm SF₆, 60sccm O₂, 21 sccm SiF₄, and 5 sccm C₄F₈ as a constant gas flow rate. Thepressure was 15 mTorr=20 izbar, ICP high-frequency power was 800 W at13.56 MHz, and substrate (bias) power was 10 to 15 W at 13.56 MHz. Thebias voltage was set at 40 to 100 V. With a pulsed C₄F₈ flow, thefollowing gas flow rates were set: 40 sccm SF₆, 60 sccm O₂, 21 sccmSiF₄, and 30-60, preferably 45, sccm C₄F₈. The C₄F₈ was deliveredperiodically every 30-60 seconds, preferably every 45 seconds, for aperiod of 5 seconds each time. ICP high-frequency power in this case was800 W, and substrate power 12 W.

What is claimed is:
 1. A method for anisotropic plasma etching laterallydefined patterns in a silicon substrate, comprising the steps of:anisotropic plasma etching the laterally defined patterns in the siliconsubstrate using an etching gas to release flourine radicals in a plasma;and before the etching step or during the etching step, depositingprotective layers onto sidewalls of the laterally defined patterns, theprotective layers being formed by at least one silicon compound beingused as a secondary reactant, and using at least one of an oxide formerand a nitride former, wherein the secondary reactant does notsubstantially react with the flourine radicals released from the etchinggas, and the secondary reactant does react with at least one of oxygenfrom the oxide former and nitrogen from the nitride former, and theetching gas; wherein the etching gas, the at least one of the oxideformer and the nitride former, and the secondary reactant are eachbromine-free and chlorine-free.
 2. The method according to claim 1,wherein the etching step and the depositing step are performed separatefrom one another, the etching step being performed alternately with thedepositing step.
 3. The method according to claim 1, wherein the etchingstep is performed using a medium, the medium having a gas flow ratebetween 10 sccm and 200 sccm and a process pressure between 1 μbar and50 μbar.
 4. The method according to claim 1, further comprising the stepof: during the etching step, cooling the silicon substrate.
 5. Themethod according to claim 1, further comprising the step of: generatingan anisotropic plasma using one of a microwave irradiation and ahigh-frequency irradiation having a power level between 500 W and 2000W.
 6. The method according to claim 1, wherein the etching step isperformed simultaneous with an ion irradiation.
 7. The method accordingto claim 6, wherein the ion irradiation has an ion energy between 1 eVand 100 eV.
 8. The method according to claim 6, wherein the ionirradiation has an ion energy between 30 eV and 50 eV.
 9. The methodaccording to claim 6, wherein an ion density, an ion energy, and a ratiobetween charged particles and uncharged particles are controlledindependently of one another.
 10. The method according to claim 1,wherein the at least one silicon compound includes one of silicon oxide,silicon nitride and silicon oxynitride.
 11. The method according toclaim 10, wherein the depositing step includes depositing one of asilicon oxide layer and a silicon nitride layer onto the sidewalls. 12.The method according to claim 11, wherein the at least one siliconcompound further includes SiF₄.
 13. The method according to claim 11,wherein one of O₂, N₂O, NO, NO_(x), CO₂, and N₂ is added to the etchinggas.
 14. The method according to claim 11, further comprising the stepof: adding at least one of O₂ and N₂ to the etching gas as a reactionpartner.
 15. The method according to claim 11, wherein the etching gasprovides at least one of O₂ and N₂ for use as a reaction partner. 16.The method according to claims 11, further comprising the step of:delivering at least one SiO₂ scavenger gas for a short time period andonly periodically to the etching gas for intensive cleaning of anetching front, the at least one SiO₂ scavenger gas being bromine-freeand chlorine-free.
 17. The method according to claim 11, wherein a gasflow rate of a silicon tetrafluoride SiF₄ is between 10 sccm and 50sccm.
 18. The method according to claim 11, wherein a gas flow rate ofO₂ is between 10 sccm and 100 sccm.
 19. The method according to claim11, wherein the etching gas includes one of SF₆ and NF₃.
 20. The methodaccording to claim 19, wherein a gas flow rate of the sulfurhexafluoride SF₆ is between 20 sccm and 200 sccm.
 21. The methodaccording to claim 11, further comprising the step of: continuouslyadding at least one SiO₂ scavenger gas to the etching gas, the at leastone SiO₂ scavenger gas being bromine-free and chlorine-free.
 22. Themethod according to claim 21, wherein the at least one SiO₂ scavengergas includes one of CHF₃, CF₄, C₂F₆, C₃F₆, and C₄F₈.
 23. The methodaccording to claim 21, wherein a gas flow rate of the at least one SiO₂scavenger gas is between 2 sccm and 10 sccm.
 24. The method according toclaim 23, wherein the at least one SiO₂ scavenger gas includes C₄F₈. 25.The method according to claim 24, further comprising the step of: addingC₄F₈ to the etching gas periodically every 30 seconds to 60 seconds fora period of 5 seconds each time, C₄F₈ having a flow rate between 30 sccmand 60 sccm.